The present invention relates to a venting device for relieving pressure in response to an over-pressure condition existing within a pressure vessel. More particularly, the present invention relates to electrochemical cells capable of venting at predetermined over-pressure conditions.
Electrochemical cells, such as Lithium type batteries, undergo chemical reactions which product high pressures within the cells. For this reason, the cell cans of Lithium type batteries are generally comprised of a metal material capable of withstanding such high pressures and attack from the enclosed reactive chemicals. However, unusual conditions such as external shorts, presence of low impedance internal shorts or inadvertent exposure to excessive temperatures can cause increases in cell pressures which, if not relieved, could cause a cell to explode. Generally, a mechanical safety vent is thus incorporated in Lithium type cells as a safeguard against the catastrophic results which could occur under such conditions.
Prior to the present invention, cell types which have utilized vents have been unitary in structure with a portion of the side wall of the can deliberately weakened by thinning it so that an opening would be formed in the side wall of the can at elevated pressures to allow for pressure relief. However, such vents are dependent upon elastic stretching of the can's material, and as such, the venting characteristics cannot be accurately controlled due to various factors including curvature, thickness and material condition. For example, the pressure at which a vent is formed with such venting device is determined not by the stable elastic properties of the can's material, but by its ultimate plastic deformation behavior which is dependent upon composition and the mechanical and thermal history of the can.
Numerous devices have been suggested for venting pressure vessels. Vents in general have been constructed as one time safety devices in which pressurized material is permitted egress from an enclosed space. U.S. Pat. No. 3,918,610 disclosed one such vent for an aerosol container consisting of a "bridge" area which spans a hollow grove. The bridging "rib" has across it a score or weakening line extending transversely to the direction of the bridge. When the pressure becomes excessive within the aerosol container, the rib will fracture on the score line to vent the pressure from the container. Although this venting device has been used, it obviously requires intricate and correspondingly expensive manufacturing of the "bridge" area.
Another invention for venting a pressure vessel is disclosed in U.S. Pat. No. 3,463,351. This invention utilizes a concave-convex rupture disc containing lines of weakness on either surface and a member attached to its concave surface to achiever full opening without fragmentation.
The concave-convex disc vent, by its very nature, is inefficient in its utilization of space. Additionally, this vent, as disclosed, requires that the rupture disc be held in place between two pipe flanges. Such an application would be difficult, as well as expensive, to apply in a pressure vessel of limited dimensions, as for example in a Lithium cell.
The present invention discloses an improved safety vent which overcomes the drawbacks of the art of record in venting electrochemical cells, and in particular, Lithium cells.